1. FIELD OF THE INVENTION
The present invention relates to a touch signal probe suitable for being a coordinate-measuring machine or a machine tool for measuring configuration of a workpiece. More specifically, it relates to a highly sensitive vibrating touch signal probe requiring low measurement power.
2. DESCRIPTION OF RELATED ART
A height gauge (linear measuring machine), a coordinate measuring machine and a contour measuring machine are known as a measuring machine for measuring configuration and dimension of a workpiece. For measuring coordinate and position, a touch signal probe is used to the measuring machine for detecting contact of the workpiece.
One detection mechanism uses a substantially cylindrical stylus having a contact portion to be abutted to the workpiece at the pointed end thereof, and a vibrating/detecting means for vibrating the stylus and detecting change of the vibration accompanied by the contact of the contact portion to the workpiece. In this arrangement, a touch trigger signal is transmitted when the vibration is damped to a detection trigger level and a coordinate value thereat is read.
In the vibrating touch signal probe, a radial arrangement (forming a plurality of stylus to a probe in a ramifying manner) is desirable for broadening the applicable range, and arranging the stylus crosswise is effective, for example.
One example of the probe having crosswise-arranged stylus is shown in FIG. 12. In the Figure, the vibrating touch signal probe has a pair of stylus support 3 fixed to a pointed end of the probe body 5 as a probe axis. The pair of the stylus support 3 is joined to orient in X and Y axis direction respectively, and a stylus 2 is protruded on both sides thereof. An upper portion of respective stylus support 3 has a piezoelectric element 4 as a vibrating/detecting means provided along the respective styluses 2. Each of the stylus 2 is vibrated by the piezoelectric element 4 and the contact ball 2a on the pointed end thereof is abutted to a workpiece, thereby detecting a change in restraining condition of the vibration (conventional art 1).
According to the conventional art 1, since two pairs of linear stylus support 3 are combined and the stylus is not disposed coplanarly, it is inconvenient in practical use and size reduction thereof is difficult.
On the other hand, a touch signal probe having a block-shaped stylus support, a piezoelectric element as vibrating/detecting means disposed to locating projection projectingly provided to four corners of respective upper and lower surfaces of the stylus support, and a symmetric first and second stylus respectively oriented in X and Y axis direction at a center of each side of the stylus support, thereby arranging radially, has been developed (Japanese Patent Laid-Open No. Hei 10-176902: conventional art 2).
According to the conventional art 2, since the symmetric first and second styluses oriented respectively in X and Y axis are disposed at the center of respective sides of the stylus support, there is no inconvenience as in the conventional art 1. However, since a reciprocating vibration is caused to both of the X and Y axis by a single piezoelectric element, a total of four vibration system is combined, resulting in difficulty in raising Q value of a resonant vibration as compared to the conventional art 1.
FIGS. 13(A) and 13(B) are graphs showing a relationship between a frequency and amplitude in the aforesaid conventional art 1 and 2. FIG. 13(A) shows small Q value case and FIG. 13(B) shows large Q value case. Amplitude difference D (amplitude change by contact) between amplitude at a resonant point in a non-contact state and amplitude at the resonant point after contact is larger when the Q value is large than when the Q value is small. Accordingly, it can be observed that the Q value of the vibration at the resonant state is a significant factor which directly controls sensitivity of a vibrating touch signal probe, in which the change in resonance by contact is used as detection principle of detection.
As described above, the touch signal probe having radial arrangement as in the conventional art 2 is inferior in sensitivity to the conventional art 1 having the same stylus length, and the detection response time can be widely varied.
Furthermore, when the stylus contacts at a high speed (more than 10mm/sec, at present), the touch signal prove is vibrated by an impact in contact, resulting in unpredictable disorder of detected amplitude change.
When the stylus touches the workpiece at a high-speed, detected vibration displacement is regularly decreased from initiation of contact as shown in FIG. 14(A). On the other hand, newly generated vibration waveform can be detected by the impact as shown in FIG. 14(B). These signals are combined to be the detection signal of the stylus.
FIG. 14(C) is a graph showing relationship between amplitude and time, the amplitude representing the signal detected by the stylus and converted to DC level. In FIG. 14(C), the time before reaching detection level of the touch trigger signal differs between a case in which the vibration by the impact is applied in equal phase (shown in solid line) and a case in which the vibration by the impact is applied in inverse phase (shown in dotted line). The time difference becomes dispersion error C of detection response time, which causes detection error.
Accordingly, the dispersion of the detection response time increases in accordance with the increase of the contact speed. Incidentally, the phenomenon inevitably occurs in common to all of the conventional vibrating touch signal probes. Since the frequency of the vibration caused by the contact is intrinsic frequency of the vibrator composed of the stylus and the stylus support, it is difficult to separate the signal component on account of the principle in which the resonant frequency is used for vibration and detection.
A touch signal probe of a modification of the conventional art 2 is shown in FIG. 15. A probe support 50 supports a block-shaped stylus support 51. Locating projections 52 are projectingly disposed on four comers of upper and lower side of the stylus support 51. Piezoelectric elements 53 as vibrating means and detecting means are disposed on the respective locating projections 52 and symmetric first and second styluses 54 respectively oriented in X and Y-axis directions are disposed to a center of respective sides of the stylus support. Since the entire body including the stylus support 51 and the plurality of the styluses 54 is a vibrator forming one vibration system, the above-structured touch signal probe can be simply and easily assembled and stable resonance characteristic can be obtained.
The piezoelectric element 53 uses conventional linear wiring pattern for ultrasonic touch sensor. Specifically, a part of one piezoelectric element 3 is used as a vibrating electrode 53A and the other part of the piezoelectric element 53 is used as a detecting electrode 53B. The electrode 53A and 53B are disposed aligning a longitudinal direction of the styluses 54 opposed with each other. A vibrating circuit and a control circuit shown in FIG. 16 is connected to the piezoelectric 53. In the Figure, the vibrating circuit is composed of a driving circuit 55 for applying vibrating electric current to the vibrating electrode 53A and a power supply 56 connected to the driving circuit 55. The control circuit is composed of an amplitude-DC level converting circuit 57 for converting amplitude of the detected signal detected by the detecting electrode 53B into DC (direct current) level and a touch trigger signal generating circuit 58 for generating a touch trigger signal in accordance with the signal outputted from the amplitude-DC level converting circuit 57 (conventional art 3).
In the conventional art 3, the vibration detecting direction of the vibrator of the piezoelectric element 53 is basically monoaxial. However, since the piezoelectric element 53 expands and contracts in accordance with vibration in intersecting direction, the vibration of the vibrator oriented in the intersecting direction can be detected.
However, though the vibration in basic direction can be sufficiently detected, the vibration in a direction intersecting the basic direction can be insufficiently detected on account of indirect signal detection, resulting in dispersion in detection property of respective styluses. Accordingly, it may be preferable to provide a vibration detecting means to each vibrator in spite of complexity of the structure.
In the conventional art 3, for ensuring sufficient output level of the signal sent to the amplitude-DC level converting circuit 57, a detection signal S is generated by analogue-adding a detection signal S1 detected by one piezoelectric element 53 and another detection signal S2 detected by the other piezoelectric element 53, as shown in FIG. 17.
Since the detection signal S is generated by analogue-adding each detection signal S1 and S2, direct waveform fluctuation occurred to one detection signal is flattened by the other detection signal, thereby blunting waveform fluctuation of the detection signal S immediately after contact, as shown in FIG. 18.
More specifically, when only the individual detection signal S1 is amplitude-DC-level-converted, it takes detection delay t1 for the signal S1 to initially damp to detection trigger level of the touch trigger signal from initiation of contact. When only the individual detection signal S2 is amplitude-DC-level-converted, it takes detection delay t2 for the signal S2 to initially damp to detection trigger level of the touch trigger signal from initiation of contact. On the other hand, when the detection signal S adding the detection signal S1 and S2 is amplitude-DC-level-converted, detection delay t for the signal S to initially damp to detection trigger level of the touch trigger signal from initiation of contact is larger than t1 and t2, since the waveform of the detection signal S is blunted by adding S1 and S2 of different system.
In addition, since the detected signal S is of single system, initial behavior of the signal waveform does not reach the detection trigger level if the behavior of the signal waveform changes, thereby losing great response time before reaching the detection trigger level by the subsequent behavior as shown in FIG. 19.
FIG. 19 shows error factor of repeating measurement. The detection signal S of the least delay has detection delay t from the start of contact to initially damp to the detection trigger level of the touch trigger signal. On the other hand, the waveform of the same system signal can be fluctuated in repeating measurement. The detection delay of the detection signal Sxe2x80x2 having fluctuated waveform is txe2x80x2. The time difference (txe2x80x2-t) composes dispersion error of detection delay.
An object of the present invention is to provide a touch signal probe which can measure highly accurately being made in cross-shaped formation and allows size reduction.
Accordingly, the vibrator is vibrated by a mode resonance having high Q value when the cross-formed vibrator is resonantly vibrated.
Specifically, a touch signal probe according to the present invention is for detecting contact to a workpiece by a change in vibration in contacting the workpiece. The touch signal probe has: a stylus support of which center is an origin of X, Y and Z-axis orthogonal with each other; a vibrator provided to the stylus support and having a stylus attached along at least one of the X-axis and Y-axis; and a vibrating/detecting means disposed to a side of the stylus support perpendicular to the Z-axis of the stylus support for vibrating the vibrator at a predetermined frequency and for detecting vibration change caused when the stylus contacts the workpiece. The touch signal probe is characterized in that the vibrator is vibrated at a vibration frequency equal to either one of primary intrinsic frequency xcfx891 or secondary intrinsic frequency xcfx892 (xcfx892xe2x89xa0xcfx891) of the vibrator, and that the vibration change caused when the stylus contacts the workpiece is amplified by superposing vibration component of the other one of primary intrinsic frequency xcfx891 or secondary intrinsic frequency xcfx892 in the vibration caused when the stylus contacts the workpiece.
Incidentally, the primary intrinsic frequency xcfx891 and the secondary intrinsic frequency xcfx892 is unique to the vibrator.
According to the present invention, the touch signal probe is moved to touch the workpiece in measurement while the vibrator is vibrated at a vibration mode having high Q value by the vibrating/detecting means, at a frequency equal to the secondary intrinsic frequency xcfx892, for instance, to make resonance. Since the vibration at the secondary intrinsic frequency (secondary mode resonance) has small response amplitude notwithstanding high Q value, a vibration component of the primary intrinsic frequency xcfx891 is superposed to amplify the vibration change.
By the above vibrator and vibrating direction, since the magnitude of amplitude at the pointed end of the stylus of the vibrator is not decreased as compared to the vibration at the primary intrinsic frequency even in the vibration at the secondary intrinsic frequency, the vibration of the stylus can be sure to restrained by the contact to the workpiece, thereby securely causing vibration change by the contact. Accordingly, the vibration of the stylus is securely restricted and is accurately detected by the vibrating/detecting element.
Furthermore, since the stylus can be disposed coplanarly, the size of the touch signal probe can be reduced even when the stylus is supported by the stylus support crosswise.
The touch signal probe according to the present invention may further include a beat signal component sampling means for detecting a beat signal generated by superposing the primary intrinsic frequency xcfx891 and the secondary intrinsic frequency xcfx892.
According to the above arrangement, since beat cycle of the beat signal is determined by a difference between the two frequencies, the frequency of the beat signal component is quite different from the intrinsic frequency. Accordingly, the generated beat signal component can be easily sampled by a filter or a resonator, thereby positively detecting superposition of the primary intrinsic frequency xcfx891 and the secondary intrinsic frequency xcfx892 to positively detecting the vibration generated in contact. Therefore, S/N (signal-to-noise ratio) can be improved.
The stylus may preferably include first pair of stylus attached along the X-axis and symmetrically disposed with the origin positioned therebetween and second pair of stylus attached along the Y-axis and symmetrically disposed with the origin positioned therebetween.
According to the above, a cross stylus arrangement having styluses respectively extending from four sides of the stylus support in X and Y-axis direction can be attained.
On the other hand, the stylus may also be preferably provided to the stylus support along either one of the X-axis and Y-axis, and a balance member may be provided to a portion of the stylus support along either one of the X-axis or the Y-axis of the stylus support having no stylus thereto, the balance member being shaped to be dynamically equivalent to the stylus.
Accordingly, an accurate measurement is possible even in an irregularly-structured vibrator having styluses along only either one of the X-axis or the Y-axis on the stylus support, since the balance member balances with the stylus in vibration.
Another object of the present invention is to provide a touch signal probe which can reduce detection delay and improve detecting power dispersion between styluses to eliminate measurement error.
For the object, a plurality of detecting means (not vibrating means) is disposed correspondingly to a plurality of styluses in the present invention and a plurality of detection signal respectively obtained by the detecting means is independently used without combining with each other.
Specifically, a touch signal probe according to the present invention is for detecting contact to a workpiece. The touch signal probe includes: a stylus support provided to a probe support; a vibrator radially and projectingly formed to the stylus support, the vibrator including a plurality of styluses having contact portion for touching workpiece at a pointed end thereof; a vibrating means disposed to the stylus support for vibrating the stylus at a frequency approximately coincident with an intrinsic frequency of the vibrator; a detecting means provided to the stylus support for detecting the vibration which changes when the contact portion touches a workpiece; and a control means for arithmetically processing a detection signal detected by the detecting means to generate a contact trigger signal. The touch signal probe is characterized in that the detecting means is disposed to the stylus support in plural respectively corresponding to the plurality of styluses, and the control means separately uses a plurality of detection signal obtained by the plurality of detecting means without combining with each other so that the vibration change in accordance with the contact of the contact portion to the workpiece can be detected without depending on styluses not touching the workpiece and a direction in which the contact portion touches the workpiece.
According to the present invention, when the touch signal probe is moved so that the contact portion touches the workpiece in measurement, the vibrator is vibrated by the vibrating means to make resonance. When the contact portion of the stylus touches the workpiece while the vibrator is resonated, the detection signal detected by the detecting means varies. A touch trigger signal is generated by being processed by the control means.
Accordingly, the vibrating and detecting can be simultaneously conducted in all the direction of the stylus so that the detection can be done under the same condition whichever contact portion of the plurality of styluses contacts the workpiece. Therefore, the dispersion of detection power between each stylus can be cancelled.
Furthermore, since the detection signal detected by respective detecting means can be processed independently without adding, the detection signal from each detection signal does not interfere with each other. Accordingly, the signal which changes sharply can be detected without blunting, thereby shortening detection delay.
In the present invention, the control means may preferably generate separate touch trigger signal individual to each processing system by separately signal-processing the plurality of detection signals independently obtained by the plurality of detecting means and selecting first-arrived signal of the individual touch trigger signal generated from all of the processing system to make an overall touch trigger signal, thereby detection delay from an initiation of the contact portion to the workpiece to a generation of the overall touch trigger signal does not depend on styluses not touching the workpiece and the direction in which the contact portion touches the workpiece.
According to the above arrangement, since the first generated touch trigger signal within the plurality of touch trigger signal generated by independently signal-processing by respective signal system is used as a single touch trigger signal, the detection delay of sensor can be minimized even when waveform of the detection signal in each system is varied in repeated measurement, thereby reducing the detection delay.
In other words, when the measurement is repeated, the system detection signal having shortest arriving time is determined for generating contact trigger signal. Even if the signal waveform of the system is changed, the touch trigger signal is generated by another system detection signal having the second shortest arriving time, thereby always minimizing the detection delay.
The plurality of detecting means may preferably be independently disposed on first side of the stylus support, and the vibrating means may be disposed in plural to second side of the stylus support opposite to the first side with the stylus positioned therebetween.
According to the above arrangement, since the detecting means and the vibrating means are oppositely disposed on both sides of the base end of the stylus, the detecting means and the vibrating means can be closely disposed on both sides of the base end of the stylus, thereby securely vibrating the stylus adjacently to the stylus by the vibrating means and detecting the change of the vibration by the detecting means.